JEE Main & Advanced JEE Main Paper (Held On 8 April 2017)

  • question_answer
    The locus of the point of intersection of the straight lines,\[tx-2y-3t=0\]\[x-2ty+3=0\]\[(t\in R),\]is:         [JEE Online 08-04-2017]

    A)  A hyperbola with the length of conjugate axis 3

    B)  a hyperbola with eccentricity\[\sqrt{5}\]

    C)  an ellipse with the length of major axis 6

    D)  an ellipse with eccentricity\[\frac{2}{\sqrt{5}}\]

    Correct Answer: A

    Solution :

                    \[tx-2y-3t=0\]                 \[x-2ty+3=0\]                 \[\frac{\begin{align}   & tx-2y-3t=0 \\  & tx-2{{t}^{2}}y\underline{+}3t=0 \\ \end{align}}{y(2{{t}^{2}}-2)=6t}\]             \[\frac{\begin{align}   & {{t}^{2}}x-2ty-3{{t}^{2}}=0 \\  & -x-2ty\underline{+}3=0 \\ \end{align}}{({{t}^{2}}-1)x=(3{{t}^{2}}+1)}\]                 \[y\frac{6t}{2{{t}^{2}}-2}=\frac{3t}{{{t}^{2}}-1}\]               \[x=-3\sec 2\theta \]                 \[2y=3(-tan2\theta )\]                 \[{{\sec }^{2}}2\theta -{{\tan }^{2}}2\theta =1\]                 \[\frac{{{x}^{2}}}{9}-\frac{{{y}^{2}}}{9/4}=1\]                 \[{{a}^{2}}=9;\]                 \[{{b}^{2}}=9/4\]                 \[\lambda (T.A)=6\]        \[;{{e}^{2}}=1+\frac{9/4}{9}=1+\frac{1}{4};e\,e=\frac{\sqrt{5}}{2}\]


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